Because bond breaking is an endothermic process and bond forming is an exothermic process, enthalpy of a reaction can be calculated using bond energies. ΔH = ɛ bonds broken - ɛ bonds formed Before adding, the bond energies must be multiplied by the number of that bond present AND the coefficient from the balanced chemical equation. When writing the balanced equation, the structure of each species must be drawn to determine the types of bonds present.
Hess's Law Hess's Law states that the heat of a reaction may be determined algebraically by adding the ΔH values for the related reactions. When added, these reference reactions must give the desired or target reaction. When determining ΔH, it may be necessary to reverse and/or adjust molar ratios by multiplying by a factor. Follow these rules when calculating for ΔH of the target reaction. 1. If a reaction must be reversed, the sing of the ΔH for that reaction must be changed (negative to positive and vice versa) 2. When adjusting molar ratios by multiplying, all coefficients in that equation must be multiplied by the same factor, as well as the ΔH value for that reaction. (basically, whatever is done to the equation must be done to its corresponding ΔH value.)
Ms. Wilson put up two videos that help explain Hess's Law as ISP work. Those videos can also help explain because I don't have visuals to put onto the wiki.
ΔH = ɛ bonds broken - ɛ bonds formed
Before adding, the bond energies must be multiplied by the number of that bond present AND the coefficient from the balanced chemical equation. When writing the balanced equation, the structure of each species must be drawn to determine the types of bonds present.
This is a link to a video that will also explain how to use bond energies.
https://www.youtube.com/watch?v=ih11AH0lEZY
Hess's Law
Hess's Law states that the heat of a reaction may be determined algebraically by adding the ΔH values for the related reactions. When added, these reference reactions must give the desired or target reaction. When determining ΔH, it may be necessary to reverse and/or adjust molar ratios by multiplying by a factor.
Follow these rules when calculating for ΔH of the target reaction.
1. If a reaction must be reversed, the sing of the ΔH for that reaction must be changed (negative to positive and vice versa)
2. When adjusting molar ratios by multiplying, all coefficients in that equation must be multiplied by the same factor, as well as the ΔH value for that reaction.
(basically, whatever is done to the equation must be done to its corresponding ΔH value.)
Ms. Wilson put up two videos that help explain Hess's Law as ISP work. Those videos can also help explain because I don't have visuals to put onto the wiki.
https://www.youtube.com/watch?v=wH0jn0qb3J4
https://www.youtube.com/watch?v=9oaNmEqY-ss